The present invention relates to a toner for image formation by developing electrostatic images or toner jetting, particularly a toner capable of providing high-definition fixed images even when obtained through a heat-pressure fixing means using no or only a limited amount of oil for preventing high-temperature offset. The present invention also relates to a full-color image forming method using such a toner.
Full color copying machines proposed in recent years have generally adopted a process wherein four photosensitive members and a belt-form transfer member are used, electrostatic images formed on the photosensitive members are developed with a cyan toner, a magenta toner, a yellow toner and a black toner, respectively, to form respective toner images on the photosensitive members, and the toner images are successively transferred onto a transfer(-receiving) material conveyed along a straight path between the photosensitive members and the belt-form transfer member to forma full-color image; or a process wherein a transfer(-receiving) material is wound about the circumference of a transfer member with an electrostatic force or a mechanical force exerted by e.g., a gripper, and a development-transfer cycle is repeated four times to form a full color image on the transfer material.
Toners used in such a full-color copying machine are required to exhibit an improved color reproducibility and cause sufficient color mixing in a heat-pressure fixing to provide a full color image with good transparency as required in overhead projector (OHP) images. Compared with an ordinary black toner for mono-chromatic copying machines, a toner for full-color image formation may preferably comprise a relatively low-molecular weight binder resin exhibiting a sharp-melting characteristic. However, a toner comprising such a sharp-melting binder resin is liable to cause a problem of high-temperature offset because of low self-cohesion of the binder resin at the time of toner melting in the heat-pressure fixing step.
For an ordinary black toner for monochromatic copying machine, a relatively high-crystalline wax as represented by polyethylene wax or polypropylene wax has been used as a release agent in order to improve the anti-high-temperature offset characteristic at the time of fixation, as proposed in Japanese Patent Publication (JP-B) 52-3304, JP-B 52-3305 and JP-B 57-52574. When such a high-crystallinity wax is used in a toner for full-color image formation, however, the fixed toner image is liable to have inferior transparency, thus providing a projected image with lower saturation and brightness when projected as an OHP image, because of the high crystallinity and difference in refractive index from an OHP sheet material of the wax.
In order to solve such problems, some toners having a specific storage modulus or viscoelasticities have been proposed.
For example, Japanese Laid-Open Patent Application (JP-A) 11-84716 and JP-A 8-54750 have proposed a toner having a specific storage modulus at 180xc2x0 C. or 170xc2x0 C. The toner has tho low a viscosity and has left room for improvement in respect of storage stability in a high temperature environment, when considered as a color toner expected to exhibit a combination of low-temperature fixability and high-temperature offset characteristic, good fixability when fixed by a heat-pressure fixing means using no or only a limited amount of oil for high-temperature offset prevention, and sufficient color mixing characteristic.
JP-A 11-7151 and JP-A 6-59504 have proposed a toner showing specific storage modulus Gxe2x80x2 at 70-120xc2x0 C. and specific loss modulus Gxe2x80x3 at 130-180xc2x0 C. The toner is not satisfactory in respects of sufficient storability in a high temperature environment, performance of stably providing high-quality images in continuous formation of a large number of image products and stable chargeability and developing performance in various environments.
JP-A 5-249735, JP-A 7-92737, JP-A 7-234542, JP-A 7-295298, JP-A 8-234480, JP-A 8-278662 and JP-A 10-171156 have also proposed toners having specific viscoelasticities. However, then toners still have left room for improvement regarding fixing performances, storage stability and transparency for OHP use (i.e., for providing transparencies used in OHP""s (overhead projectors).
In order to solve the above problem, the use of a nucleating agent together with a wax for lowering the wax crystallinity has been proposed in Japanese Laid-Open Patent Application (JP-A) 4-149559 and JP-A 4-107467. The use of waxes having a low crystallinity has been proposed in JP-A 4-301853 and JP-A 5-61238. Montan wax has relatively good transparency and a low-melting point, and the use of montan waxes has been proposed in JP-A 1-185660, JP-A 1-185661, JP-A 1-185662, JP-A 1-185663 and JP-A 1-238672. However, such waxes cannot fully satisfy all the requirements of transparency for OHP use, and low-temperature fixability and anti-high temperature offset characteristic at the time of heat-pressure fixation.
For this reason, it has been generally practiced to minimize or omit such a wax or release agent in an ordinary color toner and apply an oil, such as silicone oil or fluorine-containing oil onto a heat-fixing roller so as to improve the anti-high temperature offset characteristic and the transparency for OHP use. However, according to the measure, the resultant fixed image is liable to have excessive oil on its surface, and the oil is liable to soil the photosensitive member by attachment and swell the fixing roller to shorten the life of the roller. Further, the oil has to be supplied to the fixing roller surface uniformly and at a controlled rate in order to prevent the occurrence of oil lines on the fixed image, and thus tends to require an increase in overall size of the fixing apparatus.
Accordingly, there is a strong desire for a toner which can effectively suppress the occurrence of offset when used in a heat-pressure fixing means omitting or minimizing the use of such an oil for preventing high-temperature offset, and can also provide fixed images with an excellent transparency.
A generic object of the present invention is to provide a toner having solved the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide a color toner exhibiting excellent transparency for OHP use and anti-high-temperature offset characteristic.
Another object of the present invention is to provide a toner with excellent low-temperature fixability.
Another object of the present invention is to provide a toner with excellent storability, heat-resistance and anti-blocking property.
Another object of the present invention is to provide a toner with stable chargeability which is little affected by a change in environmental conditions of temperature and humidity.
A further object of the present invention is to provide a full-color image forming method capable of providing full-color images with excellent color mixing characteristic and color reproducibility by using substantially no fixing oil.
According to the present invention, there is provided a toner, comprising: at least a binder resin, a colorant and a wax, wherein
the toner has viscoelasticity including: a storage modulus at 80xc2x0 C. (Gxe2x80x280) in a range of 1xc3x97106-1xc3x971010 dN/m2, storage moduli at temperatures of 120-180xc2x0 C. (Gxe2x80x2120-180) in a range of 5xc3x97103-1xc3x97106 dN/m2, and loss tangents (tan xcex4=Gxe2x80x3/Gxe2x80x2 as a ratio between Gxe2x80x3 (loss modulus) and Gxe2x80x2 (storage molecules)) including a loss tangent at 180xc2x0 C. (tan xcex4180) and a minimum of loss tangents over a temperature range of 120-180xc2x0 C. (tan xcex4min) satisfying 1xe2x89xa6tan xcex4180/tan xcex4min, and
the toner exhibits a thermal behavior providing a heat-absorption curve according to differential scanning calorimetry (DSC) showing a maximum heat-absorption peak temperature in a range of 50-110xc2x0 C. in a temperature range of 30-200xc2x0 C.
According to the present invention, there is further provided an image forming method, comprising:
(A) an image forming cycle including:
a step of forming an electrostatic image on an image bearing member,
a step of developing the electrostatic image with a color toner to form a color toner image on the image bearing member, and
a step of transferring the color toner image onto a transfer material via or without via an intermediate transfer member,
(B) a process of repeating the image forming cycle (A) four times by using first to fourth color toners, respectively, to form superposed first to fourth color toner images on the transfer material, and
(C) a step of fixing the superposed first to fourth color toner images on the transfer material under application of heat and pressure to form a fixed full-color image on the transfer material, wherein
the first to fourth color toners are selected successively in an arbitrary order from the group consisting of a cyan toner, a magenta toner, a yellow toner and a black toner,
each of the cyan, magenta, yellow and black toners comprises at least a binder resin, a wax and a corresponding colorant selected from the group consisting of a cyan colorant, a magenta colorant, a yellow colorant and a black colorant,
the toner has viscoelasticity including: a storage modulus at 80xc2x0 C. (Gxe2x80x280) in a range of 1xc3x97106-1xc3x971010 dN/m2, storage moduli at temperatures of 120-180xc2x0 C. (Gxe2x80x2120-180) in a range of 5xc3x97103-1xc3x97106 dN/m2, and loss tangents (tan 6=Gxe2x80x3/Gxe2x80x2 as a ratio between Gxe2x80x2 (loss modulus) and Gxe2x80x2 (storage molecules)) including a loss tangent at 180xc2x0 C. (tan xcex4180) and a minimum of loss tangents over a temperature range of 120-180xc2x0 C. (tan xcex4min) satisfying 1xe2x89xa6tan xcex4180/tan xcex4min, and
the toner exhibits a thermal behavior providing a heat-absorption curve according to differential scanning calorimetry (DSC) showing a maximum heat-absorption peak temperature in a range of 50-110xc2x0 C. in a temperature range of 30-200xc2x0 C.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.